Your sens should depend on ur aiming skills ( having a low sensitivity is a big handicap to u when it comes down to aiming, in my days of cs the best snipers in the world used sensitivities of like 0.4, they barely missed ) , if u aint that good at aiming in games like cs, quake3 etc play with low sensitivities ( below 2 in cs ) until u get really good at aiming and then slowly move it up to a point where u can still aim good but also have speed advantage of having a high sensitivity ( allowing u to quickly pick off people u see on the edje of ur screen and do quick 180 degree turns etc ).
Also change ur sensitivity to suit the game ur playing, eg if ur playing a super fast paced game like quake3 or UT, UT2004, TFC and esf melee battles u should notch up ur sensitivity and sacrifice abit of aim to compensate for the pace of the game.
For CS i use a sens between 3 and 6
For ESF and TFC i use a sens of 10-15. ( esf 15 because huge hitboxes)
For UT,UT2004, Quake 3 i use almost max sens.